I mean this in both ways. Ones that are slooow reads, and the kind that can do harm to you if they fall on you.
Right now I'm reading The Vikings by Robert Ferguson. It has 451 pages, including notes and index. While I've been reading this I've taken breaks and read a few light books.
First the heavy one. I'm now about halfway through it, and it's definitely a worthwhile read, though I must admit I've had to look up some words in my big dictionary. Ferguson writes with humor and precision, and though I've read other books on the subject, this is the best. The chapters, in a way, are stand alone. Some totally captured me. Others, such as Across the Baltic, were a struggle. Though there are maps in the book, I had to get out my atlas to figure things out. For a largely illiterate people, they left some fascinating traces. I remember standing in Maes Howe on Mainland, Orkney, staring at the runes left behind by Vikings. Our guide said most of them were pretty much what you'd find on the walls of a men's room. One tells us "Ingegerth is most beautiful..." This is a great, exciting read...though heavy.
On the lighter side....
I just finished Michael Crichton's last book, Pirate Latitudes. We're told the finished manuscript was found after his death. I doubt he had really finished it, unless he was really under the influence of Pirates of the Caribbean. I was with him until he introduced the giant squid.